We have investigated the effect of cellular density on the expression of the low density lipoprotein (LDL) receptor in cultured human skin fibroblasts. In addition, LDL receptors and cholesterol have been measured in single fibroblasts utilizing quantitative fluorescence microscopy (i.e., microfluorometry). Morphologic studies of the formation of cholesterol containing lipid droplets in fibroblasts exposed to human low density lipoproteins are continuing.